Brushing gelcoat

[Graham_Wright]

Any tips to achieving a decent finish on brushed gelcoat would be very welcome.

Although on the deck and even in this weather, I can keep it warm enough to cure but I am having trouble losing the brush marks.

I have tried warming the resin;- how warm can I go?

I believe I can diltue with ordinary polyester resin.

Do I lay it on thick; do I brush out?

Help please. /forums/images/graemlins/confused.gif

 

[Avocet]

Maybe try a roller?

I have used stuff in the past called "Durabuild" which is (effectively) a sprayable gelcoat that can be sprayed with a gravity-feed gun if you have access to a compressor and some means of protection from the styrene fumes.

 

[Talbot]

adding gelcoat into normal polyester resin creates what is known as a flow coat - suitable for brushing on, where the majority of the brush strokes will disappear. I is much thinner, but thinness can be regulated by prportion of gelcoat to resin.

 

[boatbuilder]

The solvent for polyester gel is styrene , but be aware that to get gel to cure you have to add wax at about 5%.
You can spray it to get a good finish.
Durabild is not a gell but a high build resin that can be buffed to a gloss.
Dont forget to add butonox at 2%

 

[Graham_Wright]

I think that's what I meant by "diltue"!

 

[Graham_Wright]

[ QUOTE ]
The solvent for polyester gel is styrene , but be aware that to get gel to cure you have to add wax at about 5%.
You can spray it to get a good finish.
Durabild is not a gell but a high build resin that can be buffed to a gloss.
Dont forget to add butonox at 2%

[/ QUOTE ]

I have added wax but is the proportion critical? The gelcoat does cure but can I add too much and, if so, with what consequences?

Please xplain butonox.

 

[boatbuilder]

Butanox 50 = catalyst

Wax no more than 5%, or there abouts.

 

[Avocet]

The wax is there to make the gelcoat "think" it is up against a mould and sandwiched between the mould and the laminate. It stays tacky for ages (like months!) if it is left open to air. The wax just rises to the surface of the gel and excludes the air. If you put a bit too much in, I don't think it's a problem but of you wanted to coat it in anything else or put another layer on, it might not stick. Not sure how you get rid of it. Sanding is probably a bad idea because it will just spread the wax around. Oldsaltoz might pop into this thread sometime and hopefully he'll be familiar with the ins & outs of this. I don't know what would happen if you put on far too much though.

 

[DrBob]

In a previous life I spent quite a few years as a chemist, developing gel coats for one of the major suppliers so can provide some help.

First question is are you applying this to a flat surface or are there any vertical surfaces – it is quite important?

Gel coats are designed to be applied to a mould first and then a lay up applied behind. Brush marks are not intended to come out completely so at cold winter temps the chance of getting them out is low. It is best not to let down too much with styrene as it will mess up the thixotropy (the effect of not sagging – or “non drip”). This may not be a problem if you only need it on a flat surface but you need to ensure you get the right thickness and thinning the gel coat with styrene will make it flow away to give less thickness. Don’t apply gel coats too thickly as they will be prone to cracking.

As someone suggested letting down with a gp polyester resin may be better as the thixotropy will be decreased to a lesser degree. Bottom line is though brush marks will be difficult to get out.

Watch out for temperature. GRP cures over an extended period of time (like 48 hours +). Ok it may gel in 20 mins but the strength of a laminate (or a gel coat) builds over the next couple of days and hence you need to keep some temperature in it. I would worry over heating it to say 20 degrees to gel it and then leaving it at 0-10 deg for a couple of days – it wont cure properly. Polyester likes 15 deg +. Unlike epoxies, you cant get good low temp curing – increased catalyst or cobalt promoter only gives a shorter gel time but full strength doesn’t build up as there are too many free radicals produced at the start of the cure and this leads to incomplete cross linking.

Another problem with low temp is that air bubbles introduced when you mix in the catalyst may not come out properly and look a bit naff in the final product.

You could heat the resin to 40 deg prior to application but your gel time will be short in the can and it will cool quickly on the deck and possibly be too runny. You have to watch the temp generated during the gelling process. In colder weather the temptation is to put in more catalyst/promoter but this can give high local temps and cause colour problems – so matching a white gel coat may be difficult.

As Avocet says, wax stops the oxygen in the air inhibiting the cure of the resin hence it will be tack free. No wax and it will be tacky for months. However the best way to apply gel coat as a final coat is to cover it with melanex which can then be peeled off later. The melanex stops the air getting at it during cure and leaves a smooth surface. Great for small repairs or flat surfaces but no use otherwise.

My advice would be not to try the work until you can guarantee temps above 15 C for a reasonable time in the first few days of cure.

Hope this helps

Dr Bob

 

[Graham_Wright]

Thanks to all for very helpful responses.

The area concerned is the foredeck from the anchor/chain locker hatches forward to the bow.
For the various attachments, this area is now peppered with holes;- samson post, bow plate, anchor winch and hawsepipe, forestay attachment and two cleats.

Creating these has messed up the original surface somewhat and it needs to be restored. Due to these holes (the samson post is 100mm in diameter), it is possible to heat the foredeck area by utilising a fan heater in the chain locker which not only heats the foredeck from below (it is strengthened to around 50mm thick) but also the warm air escaping through the holes is then trapped under a cover over the whole area.

The last coat I applied was heated to around 30°C and was a big improvement on its predecessor.

The gelcoat is primarily intended to protect the GRP beneath and the "scrape area" (where the anchor chain runs across) will be covered by a renewable SS or plastic sheet so the finish is irrelevant here.

The remainder now looks reasonable.

The idea of covering the surface with melanex did occur to me but I thought I would probably make it worse. If the covering was sufficiently robust, I guess it could be "ironed" with a smooth block.

Thanks to all for your help.